In membrane exchanger devices, exchange membranes are placed parallel to one another in the form of a stack, and they are held separate from one another by separator frames. These frames are constituted by a rigid part which is called the "seal plane" and which provides sealing for the device. This part is pierced by orifices which allow fluids to flow through the stack: these orifices make up ducts for feeding and removing fluids. A perforated or expanded grid structure allows fluids to flow in the open central portion of the frame and between said central portion and one or other of the fluid flow orifices via openings referred to as "diffusers".
Each exchange zone is constituted by a stack comprising a first exchange membrane which is permeable to certain substances, a separator frame, and a second exchange membrane which is permeable to the same substances or to other substances.
There are three types of separator frame. Frames of the first type are constituted by parts that are not fixed to one another, namely: the seal plane; the expanded structure occupying the central opening; and the expanded structures corresponding to the diffusers. The building of such a stack thus requires a large number of parts to be accurately installed, which operation is very difficult and lengthly.
A second type of frame remedies this drawback in that each separator frame is constituted by a single part. Such a frame may be made, in particular, by assembling a plurality of thermoplastic sheets cut-out to the shape of the seal plane, with the expanded structure being sandwiched between them.
This second type of frame nevertheless suffers from drawbacks. In order to be able to assemble the seal plane and the expanded structure, it is necessary to select certain thermoplastic materials. Not only is the resilience of such materials generally lower than that of a traditional seal plane made of rubber, but the presence of an expanded structure within the assembly also considerably reduces its resilience, as happens in any reinforced material. This lower resilience seal plane requires higher clamping pressure to be applied to the stack in order to obtain proper sealing, thereby running the risk of buckling.
A third type of separator frame is described in the document DE-1-3 103 464. The frame is constituted by a single piece. It is obtained by molding the seal plane onto the edges of the expanded structure, with the seal plane being provided with elongate seals. In the method described, the edges of the expanded structure are subjected to compression to reduce their thickness, with the seal plane then being molded onto the compressed portion so that the non-compressed portion of the expanded structure situated in the central opening is of substantially the same thickness as the frame where it has the elongate seals.
Such crushing has the effect of flattening the component parts constituting the expanded structure, which parts are generally initially substantially circular in section. Such flattening reduces the thickness of the expanded structure in the seal plane. As a result, the loss of resilience due to the presence of the expanded structure is relatively speaking considerably less than it would be with a non-flattened expanded structure. It will be understood that an expanded structure having component parts of smaller diameter could not be used. It is important for the thickness of the seal plane in the frame to be substantially equivalent to or slightly greater than the thickness of the expanded structure since otherwise the membranes are not supported and are subjected to deformation the hinders proper operation of the exchanger device.
In Document DE-A-3 103 464, provision is made to enable the expanded structure to occupy the central opening and also the diffusers.
However, it is technically advantageous for the type of expanded structure that occupyies the diffusers to be different from the expanded structure occupying the central opening, and in particular it is desirable for the mesh to be denser and possibly also for its thickness to be greater.
This can be achieved using frames of the first type, but not with frames of the second or third above-mentioned types.
An object achieved by the present invention is to provide a method of manufacturing a one-piece frame capable of including different expanded structures in the central opening and in the diffusers, while also providing good sealing.